You can change the service for an appointment after it has been completed. In the calendar view or daily schedule just click on an appointment which is in completed state (green color). You cannot edit Invoiced appointments.
In the service details side panel that shows up with the Appointment details click on the pencil icon to edit the service and the add on.
A new card will open on top where you can change the service and price or add and remove an add on.
Here you can either change the service by selecting from the drop down or edit price of the service.
Aslo you can add or remove the addons attached to the appointment.
Once done click in save button to save the changes.
Note: The staff payment will get updated if you change the service or add or remove the add ons.